Recently, coherent illumination has been increasingly used in electrophotographic printing for image formation on photoreceptors. Unfortunately, the use of coherent illumination sources in conjunction with multilayered photoreceptors results in a print quality defect known as the "plywood effect" or the "interference fringe effect." This defect consists of a series of dark and light interference patterns that occur when the coherent light is reflected from the interfaces that pervade multilayered photoreceptors. In organic photoconductor (OPC) photoreceptors, primarily the reflection from the air/charge transport layer interface (i.e., top surface) and the reflection from the undercoat layer or charge blocking layer/substrate interface (i.e., substrate surface) account for the interference fringe effect. The effect can be eliminated if the strong charge transport layer surface reflection or the strong substrate surface reflection is eliminated or suppressed.
Many methods have been proposed to suppress the charge transport layer/air interface reflection, including roughening of the charge transport layer surface by introducing SiO.sub.2 and other particles into the charge transport layer, applying an appropriate overcoating layer and the like.
Many methods have been proposed to suppress the intensity of substrate surface reflection, e.g., coating methods such as anti-reflective coating and scattering material coating, and roughening methods such as anodization, dry blasting and wet honing. However, such methods must achieve their primary objective of eliminating substrate surface reflection without adversely impacting the electrical parameters or print quality of photoreceptors.
Patents on interference fringe effect suppression in general, or via suppression of the substrate surface reflection include U.S. Pat. No. 4,618,552 to Tanaka et al. (adding an opaque conductive layer above the ground plane), U.S. Pat. No. 4,741,918 to Nagy de Nagybaczon et al. (coating process using a buffing wheel), U.S. Pat. No. 4,904,557 to Kubo et at. (roughened photosensitive layer on top of a smooth substrate surface), U.S. Pat. No. 4,134,763 to Fujimura et. al. (grinding method to roughen the substrate surface), U.S. Pat. No. 5,096,792 to Simpson et al. (addition of antireflection layer on top of the substrate surface), U.S. Pat. No. 5,051,328 to Andrews et al. (Indium Tin Oxide transparent ground plane as the substrate), U.S. Pat. No. 5,089,908 to Jodoin et al., U.S. Pat. No. 5,069,758 to Herbert et al. and U.S. Pat. No. 4,076,564 to Fisher.
Photoreceptor substrate surfaces have been roughened by propelling ceramic and glass particles against a surface. Generally, these particles have a random particle size distribution and often have an irregular shape. For example, GB 2,224,224-A discloses an abrasive spray treatment of an electrophotographic photoreceptor substrate that hones the substrate to a satinized finish so as to eliminate interference fringe patterns and the formation of white or black spots. However, this patent application teaches away from the use of glass beads as an abrasive agent because glass beads are too spherical and tend to produce a surface that, according to this patent application, is undesirably smooth and has higher glossiness, which tends to cause an interference fringe pattern.
Because of random particle size distributions, the smaller particles used in roughening processes are often embedded into the surface of the roughened substrate. These small particles can cause black or white spots in the final electrophotographic image. Black spots occur in reversal development systems, wherein the discharged areas of an exposed photoreceptor are developed with toner particles. White spots occur in positive development systems, wherein the charged areas of an exposed photoreceptor are developed with toner particles. Also, the embedded particles are often released from the substrate during subsequent coating operations and contaminate the coating compositions that are applied to form the final photoreceptors. In addition, large particles used in the roughening process can cause large craters to form in the substrate surface, which adversely affect the thickness uniformity of the subsequently applied photoreceptor coatings.
When the particles used for roughening have an irregular shape, tiny fragments tend to break away from the particles and embed into the surface of the substrate due to the concentration of pressure during impact, particularly along the sharp edges of the particles. Moreover, small fragments that are broken away from the particles that do not lodge in the substrate surface often tenaciously adhere to the surface of the substrate due to electrostatic attraction or other phenomena and are difficult to remove prior to application of coatings. Further, control of the surface texture of the substrate is difficult, if not impossible, because the particles having an irregular shape cause the formation of an irregular texture with uneven depressions of many different sizes and shapes.
The embedded or tightly adhering fragments from the particles cause non-uniform coverage by subsequently applied coatings such as undercoating layers and charge generating layers. This, in turn, can cause black spots in the final electrophotographic images due to charge injection discharge of areas that normally retain a charge during discharged area (reversal) development. For charged area (positive) development, the defect appears as a white spot in the final xerographic image. In addition, the sharp edges on depressions can cause high fields to form during imaging, which, like the embedded or tightly adhering fragments, leads to the formation of black spots for reversal development or white spots for positive area development. Also, the deposited undercoating layers are non-uniform and uneven when applied over particle fragments or over deep depressions having sharp edges. Air bubbles can be formed when undercoating layers are applied to deep craters having sharp edges, and these air bubbles adversely affect coating uniformity.
Although materials such as ceramic materials can be shaped into a spherical shape, such shaping is complex, difficult and expensive. Moreover, ceramic materials such as those made from aluminum oxide are difficult to dispose of in an environmentally acceptable manner.